Add the StorPool block storage driver.

StorPool is distributed data storage software running on standard x86
servers.  StorPool aggregates the performance and capacity of all drives
into a shared pool of storage distributed among the servers.  Within
this storage pool the user creates thin-provisioned volumes that are
exposed to the clients as block devices.  StorPool consists of two parts
wrapped in one package - a server and a client.  The StorPool server
allows a hypervisor to act as a storage node, while the StorPool client
allows a hypervisor node to access the storage pool and act as a compute
node.  In OpenStack terms the StorPool solution allows each hypervisor
node to be both a storage and a compute node simultaneously.

To make full use of StorPool's native network communication protocol,
the Nova compute nodes will need to use the StorPool libvirt volume
attachment driver, nova.virt.libvirt.storpool.LibvirtStorPoolVolumeDriver.

DocImpact
Change-Id: I64adbca724a52771b68e4838749f8896e6e56019
Implements: blueprint storpool-block-driver
Merge "Add the StorPool block storage driver."